it normally happens with older R4s with the Zenith 281 carb but perhaps it is happening here.I HAVE IT ON 77'S
Heat radiated from the exhaust manifold heats up the carb to such an extent that the fuel in the chamber boils off and you get fuel starvation symptoms (stutters/stops). After half an hour or so with the bonnet open the carb cools down and the engine then starts and runs normally — until it heats up again. YES
I must admit I haven't yet cracked this problem on the car of mine which suffers from it. If this is indeed the cause in your case I could suggest (i) remove the radiator thermostat so the engine runs cooler, (ii) lag the exhaust manifold and carb with heat-resistant tape à la rally car. Or both. Must get round to doing this one day myself! FOR SAFETY REAQSONS I TOOK THE THERMOSTAT OFF, DOES NOT HELP
You mention that it happens after about 30 Kms (rather than miles) — thus your car is presumably a French import — these late models have quite complicated carbs which do not (in my experience with two 1985 Frog 845cc R4s) normally suffer in this way. But there is always a first time . . . .